Inherited mutations in the breast and ovarian cancer susceptibility gene, BRCA1, are associated with a high risk for developing breast and ovarian cancers. Amplification of the proto-oncogene, c-Myc, occurs in 10- 30% of human breast cancers and is associated with aggressive tumor behavior and poor prognosis. We observed a novel endogenous association of BRCA1 with Nmi (N-Myc-interacting protein) in breast cancer cells (Li et al., J Biol Chem, 2002, 277(23):20985-73). Nmi was found to interact specifically with BRCA1, both in-vitro and in-vivo, by binding to two major domains in BRCA1, amino acid residues 209-683 and 1301-1863. Nmi also functioned as an adaptor molecule to recruit c-Myc to a complex containing Nmi/c-Myc/BRCA1. The endogenous complex of Nmi/c-Myc/BRCA1 was observed in T47D breast cancer cells and MCF-10A normal breast epithelial cells. Since c-Myc can activate transcription of the human telomerase reverse transcriptase gene (hTERT), we addressed the role of BRCA1 and Nmi in modulating c-Myc induced hTERT promoter activity. While Nmi or BRCA1 alone had no effect on c-Myc induced hTERT promoter activity and telomerase activity, BRCA1 together with Nmi significantly inhibited c-Myc induced hTERT promoter activity (approximately 75% inhibition) in breast cancer cells. Two mutated forms of BRCA1, a missense (A1708E) and a nonsense (Y1853X) that have been identified in familial breast cancers, associated with Nmi and c-Myc but failed to suppress c-Myc induced hTERT promoter activity. These results suggest a novel pathogenetic mechanism whereby mutations in BRCA1, via a novel transcription factor complex containing BRCAI/c-Myc/Nmi, impair inhibition of c-Myc induced hTERT promoter activity and telomerase activity and allow sustained activation of telomerase, a key enzyme in carcinogenesis. We aim to further characterize the association of Nmi with BRCA1 and the role of the Nmi-BRCA1 complex in the transcriptional regulation and transformation of c-Myc. In addition, we will investigate the function of Nmi in mammary epithelial cells and determine the role of the novel tricomplex of BRCA1/Nmi/c-Myc in breast epithelial cells and in c-Myc induced hTERT promoter activity and telomerase activity in breast cancer cells. These studies should lead to new insights into the biology of BRCA1, and will further elucidate the molecular mechanisms that regulate the transformation of breast cancer cells.